If you start with the basic fact that innocent people have been executed for things they didn't do, the death penalty is already off the table for me. But add the high price of it, and it's time to put capital punishment to death.
Have a look at some of the statistics:
$3 million: The average taxpayer cost for trial, appeal, and prison housing for a defendant sentenced to death.
$1.1 million: Average taxpayer cost for trial, appeal, and life in prison.
147: The number of innocent people set free because of DNA evidence after being sentenced to die.
At least 50: The number of innocent people put to death after being wrongfully convicted. This number is considered unknowable, but it's an estimate based on the percentage of people on death row who have been exonerated and executed. This does not count the number of innocent people who have died in prison of suicide or other causes.